RE: [Xen-devel] Re: hvm vpt lock strangeness

["Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>]

OK, thanks.  So a per-vcpu list lock and a separate
per-domain monotonicity lock are unfortunately both
necessary (in my patch).

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
> [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxx]On Behalf Of Keir Fraser
> Sent: Friday, May 23, 2008 2:39 PM
> To: dan.magenheimer@xxxxxxxxxx; Xen-Devel (E-mail)
> Cc: Dave Winchell
> Subject: [Xen-devel] Re: hvm vpt lock strangeness
> 
> 
> Locking is required to prevent multiple concurrent access to a single
> periodic timer. This is actually implemented at vcpu 
> granularity to avoid
> lock juggling in functions which walk lists of timers. Since 
> those lists are
> held per-vcpu, the convenient locking scope is the vcpu.
> 
>  -- Keir
> 
> On 23/5/08 21:27, "Dan Magenheimer" 
> <dan.magenheimer@xxxxxxxxxx> wrote:
> 
> > Noticed this a few days ago while working on the hvm-guest-time
> > on xen-system-time patch, but forgot about it...
> >
> > Throughout xen/arch/x86/hvm/vpt.c, there are uses of a spinlock
> > called tm_lock.  But it appears that this spinlock is declared
> > and used as part of a per-vcpu data structure.  So is this
> > somehow protecting against vcpu re-entrancy (didn't think that
> > could happen) or is it supposed to be locking out one vcpu
> > against another (and not doing the job because each vcpu has
> > a separate lock)?  Or am I misunderstanding something entirely?
> >
> > If this should be a domain-wide lock, I'll spin a patch.
> > (And it might explain some of the weirder time problems?)
> >
> > Thanks,
> > Dan
> >
> > ===================================
> > Thanks... for the memory
> > I really could use more / My throughput's on the floor
> > The balloon is flat / My swap disk's fat / I've OOM's in store
> > Overcommitted so much
> > (with apologies to the late great Bob Hope)
> 
> 
> 
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